We are studying the influences of gonadal hormones on the anatomy and physiology of the excitable cells of the electric organ, electroreceptors, and the central electromotor pathway of the weakly electric fishes, Mormyridae. The electric organ produces a species-typical electrical signal, the Electric Organ Discharge (E O D), which is important for mechanisms of species and sex recognition. The characteristic properties of the EOD waveform depend upon the anatomy and physiology of the electrocytes, the excitable cells of the electric organ. We have discovered that gonadal steroid hormones can induce a male-type EOD waveform among juveniles and females of species with a natural sex difference in their EOD waveform. A special set of electroreceptors, that produce spike-like receptor potentials, show a difference in frequency tuning between natural females and, natural males or females with a hormonally-induced male EOD waveform. We will study the mechanisms underlying hormone-induced changes in EOD waveforms and electroreceptor tuning with electrophysiological, anatomical, and biochemical techniques. We have also identified, using steroid autoradiography, hormone-concentrating cells in the central electromotor pathway. These findings will be extended with additional autoradiographic studies. Since the central pathway controls the rate of electric organ excitation, i.e. the EOD rhythm, we will examine possible differences in the EOD rhythm between natural males and females, and hormone-treated specimens.